1. Field of the Invention
The present invention relates to a nonreciprocal circuit device, and particularly, to a nonreciprocal circuit device, such as an isolator or a circulator, used in a microwave band, and also to a manufacturing method of the nonreciprocal circuit device.
2. Description of the Related Art
Nonreciprocal circuit devices, such as an isolator and a circulator, have characteristics to transmit signals only in a specific direction but not in the direction opposite thereto. By using these characteristics, for example, isolators are used in transmission circuit portions of mobile communication apparatuses, such as an automobile phone and a mobile phone.
In International Publication WO 2007/046229, a nonreciprocal circuit device is disclosed in which a first central electrode and a second central electrode are wound around a substantially rectangular parallelepiped ferrite in an electrically insulated manner so as to intersect each other, a pair of permanent magnets is disposed on two primary surfaces of the ferrite to define a ferrite-magnet assembly so as to apply a direct current magnetic field to the ferrite, and side portions of the ferrite-magnet assembly mounted on a circuit board are surrounded by a yoke.
However, in the nonreciprocal circuit device disclosed in International Publication WO 2007/046229, although the periphery of the ferrite-magnet assembly is surrounded by the yoke, since a cavity is provided around the periphery, the device described above is unfavorably influenced by humidity. In addition, since the side portions of the ferrite-magnetic assembly are surrounded by the yoke, the number of components is increased, and a manufacturing process is complicated.